138 TENSION TESTING. 



ing the observations be adhered to. If one measurement 

 is missed, or incorrectly recorded, it may be that tin 

 whole of the work is wasted, and the test rendered useless. 

 The measurements may be first recorded in a rough note- 

 book, in pencil or ink, and afterwards copied into a book of 

 a more permanent character, but great care should be 

 taken that the original observations are carefully ;m<l 

 systematically recorded. 



In a tension test, such as the one we are considering,. 

 the information required is as follows: 



PRELIMINARY INFORMATION 

 Date. 



Number of test. 

 Nature of material. 

 Special marks on bar (if any). 

 Original dimensions. 



INFORMATION OBTAINED DURING THE TEST 



Loads. 



Extensions resulting from these loads. 



Permanent set (if any), after extensometer readings. 



Load at which the elastic limit occurs. 



Maximum load. 



Breaking load. 



81. Making the Test. When the necessary preliminary 

 information has been booked, and the extensometer finally 

 adjusted, the test may proceed. As elastic readings have 

 in this case to be taken, the load on the bar must be 

 increased by small steps and the corresponding extension 

 observed after each increment has been added. The 

 magnitude of the increment will depend on the nature of 

 the material and upon the size of the bar ; it must be such 

 as to give a clearly definite reading on the extensometer. 

 In the present case an increment of one-quarter or half-a- 

 ton will be suitable. 



Great care must be taken that the readings observed on 

 the extensometer be correct. The instrument should be 

 carefully watched to see that there are no disturbing 

 effects creeping in and that there are no backlash effects. 

 Readings beyond the degree of accuracy which the instru- 

 ment is capable of should never be attempted. This is a 

 mistake that is often made. 



If the modulus of elasticity is to be found with any 

 degree of accuracy, it is necessary that the readings be 



